1. Field of the Invention
This invention relates to a method of manufacturing high density fine line resolution printed circuitry and more particularly to a method in which the printed circuitry is formed on a polished, removable substrate using a high resolution photographic process.
2. Description of the Prior Art
Printed circuitry may be formed on ceramic, metal, resin and even flexible film substrates and is utilized to interconnect individual circuit components such as resistors, capacitors, inductors and semiconductor devices including complex integrated circuits. A conductive printed circuit pattern may vary from a simple pattern of radially extending conductors for connecting an integrated circuit chip to a lead frame to a highly complex multilayer pattern for interconnecting many complex circuit components.
The trend has been to require more and more complex printed circuitry with higher densities of printed conductors in order to accommodate increasingly complex circuit components in decreasing space to reduce costs and improve reliability. While line resolutions of 1 micron or better have been attained on semiconductor integrated circuits, the best resolution that is currently commercially available for printed circuitry interconnecting nonmonolithic circuit components is 5 mil wide lines on 10 mil centers. Substantial irregularities in circuit conductors have thwarted attempts to attain adequate yields because of short circuits, open circuits and high resistance narrow conductor regions. These irregularities result from a number of factors including surface roughness of the substrate, diffusion and diffraction of the exposure light as it passes through the photo mask, photoresist and irregular edge development of exposed photoresist and uneven etching of masked conductive layers.
A number of processes for forming printed circuitry on nonpermanent smooth substrates are described in U.S. Pat. Nos. 2,692,190; 2,721,822; 2,724,674; 3,181,986 and 3,350,498. However, none of these processes is capable of attaining the commercially practical, high resolution printed circuitry that is attainable with methods in accordance with the present invention.